There are a number of factors that can contribute to this condition.
Some of the potential causes are:

Increase of hormones: A woman's body releases hormones during pregnancy that allow the ligaments in her pelvic region
to soften and the joints become less constrained n preparation for giving birth. These changes may affect the abdominal
support the back normally depends on.

Added weight:With the growth of your baby and the physical changes taking place in your body to nourish and sustain the baby, your back must adjust to support additional weight.